Choosing the Right Headers
By Jim Smart/autoMedia.com
Degree of Difficulty
This oft-debated subject of choosing the right headers has been an element of bench racing for ages. Which headers make the most power? Long-tube or short-tube? Equal-length or conventional? What size tubes? Flange or ball/socket collector or somewhere in between? Ceramic coated or high-temp paint? And what is a Tri-Y header?
The following will talk about headers and offer advice on how to choose a great exhaust scavenger. In truth, there are so many variations out there. However, the basic fundamentals of header selection make it simple to select the best headers for your application.
How Will You Drive? Choosing the right header boils down to how you intend to drive your car or truck. It also boils down to fitment. Though horsepower gets a lot of publicity, torque—raw twist—is what does the grunt work of driving. It propels you from a traffic stop and gets you up to speed. And when the throttle is pinned wide open, torque should smoothly hand its duties off to horsepower at high rpm. In racing, horsepower and torque happen more at the same time at high rpm.
Header type and exhaust system design determine how torque and horsepower work together. It’s certainly true that headers affect how much power is made and where it happens. Primary tube length and size, coupled with secondary tube length and size, determines where your engine’s torque curve happens. To add confusion, exhaust system dynamics also determine where and how power happens. Pipe size, muffler and catalytic converter type—even exhaust tips—affect power output. So, how do you choose?
First, you want headers designed to fit your vehicle. If they don’t fit, nothing else matters. Primary and secondary tubes must clear the chassis, steering gear, clutch linkage, transmission bellhousing, crossmembers, any accessories, and the ground. They need to clear by at least ¼ to ½-inch to allow for thermal expansion and engine movement.
Managing Hot Gases Whether it’s a stock cast iron exhaust manifold or tubular long-tube headers, primary responsibility is to manage exhaust flow into the exhaust system. Simply put, exhaust manifolds and headers are where exhaust scavenging begins. What they do with hot gases pulsing from exhaust ports determines how your engine will perform in both power and efficiency.
When header primary tubes are too small, heat builds where you don’t want it—though you may gain torque. Because heat is destructive, you don’t want a lot of it backing up in an undersized exhaust port struggling to scavenge hot gases. This is where primary tube size is crucial. Primary tube size is determined by displacement and how aggressive your engine is. Err on the size of large versus small. Too small is destructive. Too large cheats you out of power, but won’t harm the engine.
Primary tube size selection boils down to displacement and how much power you’re expecting your engine to make. When primary tubes are too large, you lose precious torque because you lose velocity coming out of the exhaust ports. Of course this is also determined by valve overlap, that magic moment when the exhaust valve is closing and the intake valve is opening.
This turn of events creates a draw or scavenge where exiting hot gases help draw cool fuel-enriched air and fuel into the chambers. Did you know it is exhaust flow and scavenge that contribute to how fuel and air enter the chamber to be ignited? The more momentum we can create with good scavenge determines how much power we’re going to make.
Exhaust momentum comes from utilizing just the correct header design for your driving agenda. Popular logic is bigger is better, but this isn’t always true. Header tube size, length and packaging is determined by how much power you expect your engine to make and how it will be used most of the time. You want your header choice to be centered on how you will drive most of the time.
Primary and Secondary Tube Sizing The Tri-Y has long been popular in road racing, favored for its ability to vector hot gases into primary collectors then into the secondary collector. This gets you a broader torque curve, which is ideal for road racing and street driving.
Small primary tubes right off the flange may restrict exhaust flow; however, for street use, you want velocity—exhaust gas speed through those tubes. With velocity comes torque via good exhaust scavenging. If your cam profile is on the same dance floor as primary tube sizing, you will get abundant torque from velocity though you will surely lose horsepower at high rpm.
How to choose the right primary and secondary tube sizing? Size is determined by displacement and power expected. Power adders like nitrous, turbocharging, and supercharging also determine primary tube size. If you had a small-displacement V-8 and have stroked it to 347ci, primary tube size must be increased because you no longer have a 283. The most common primary tube sizes are 1-1/2”, 1-5/8”, 1-3/4”, 1-7/8”, 2”, 2- 1/8”, 2-1/4”, and 2-3/8”.
The Header Market If you go with primary tubes that are too large, you lose torque yet can gain horsepower at high rpm. Primary tubes that are too small will improve torque, lose horsepower, and trap excessive exhaust heat. Displacements of 260 to 350ci call for a 1 -½-inch primary tube. When you step up the power with a hot cam, higher compression, nitrous, turbocharging or supercharging, primary tube size needs to go up accordingly. As a matter of practice, the 1-½-inch primary works for most street applications below 350ci.
The competitive nature of the header market has resulted in much better header quality in recent years. If you have a limited budget, don’t expect high quality. You’re going to get paint instead of ceramic along with thinner flanges/tubes and shoddy welding. However, you can get a lot of bang for the buck from a couple of header manufacturers out there.
You may opt for old-fashioned header collector flanges with gaskets or ball and socket. There’s also a flange/ball combo that works quite well. We see the conventional collector flange fading away in light of better ball and socket technology.
Header Compatibility So you’ve chosen the right header for your application. However, how well do your headers, engine, and exhaust system get along? Even the best headers won’t perform if they’re plumbed into a substandard exhaust system. Header collector size and exhaust pipe size need to be compatible. Naturally, collector size and pipe size are going to be different, but they need to be close in size.
Long-tube Headers - These are the most popular with enthusiasts because long-tube headers do the best job of scavenging and making the most power, especially at high rpm. They let your engine make the most of what it has. Down side is excessive heat and potential ground clearance issues.
Short-tube Headers - or “shorties,” became common when automakers began fitting high-performance cars with them in the ‘80s. They’re an affordable aftermarket alternative to the factory cast iron scavenger, although they don’t scavenge as thoroughly as long-tube headers.